1. Field of the Invention
This invention relates to illumination of the human iris for iris imaging, as may be used for biometric identification.
2. Description of the Related Art
One of the most challenging aspects of iris imaging for biometric purposes is arranging to obtain adequate light level at the iris to obtain high signal to noise ratio (SNR) images. The iris has a low contrast and a low albedo scattering very little light back to the iris camera, requiring high illumination levels to obtain good image SNR. At the same time, eye safety limits constrain the amount of illumination that can be used. With modern CCD or CMOS imagers, the noise of the devices is already low enough that the image SNR is dominated by photon (quantum shot) noise at the intensity levels of interest. There is some room for improvement in quantum efficiency, which is currently in the range of 15 to 35% at 850 nm for the best commodity devices. Expensive scientific grade detectors can show quantum efficiency of 90% which is close to the theoretical limit but even at this level of performance detectivity is only improved by a factor of three to six. This leaves three main methods for increasing image SNR: increasing illumination level, increasing exposure time, and increasing numerical aperture.
With respect to increasing illumination levels, the eye safety limit allows for quite generous levels of illumination of the eye. However, the eye safety requirements apply at every accessible point in space. This puts significant constraints on the design of illumination systems, because in practice they are required to be eye safe even if someone looks directly into the illumination aperture. Conventional iris imaging systems predominantly work at a preferred wavelength of 850 nm. At shorter wavelengths, closer to the conventional visible wavelength band, the eye safety limitations become more stringent. Furthermore, at visible wavelengths, light of sufficient intensity may become too bright to look at, thus triggering the aversion response. At longer wavelengths than 850, the eye safety thresholds increase significantly. However, at longer wavelengths, the transparency of the iris material increases, which leads to significant changes in the image morphology. Thus, use of wavelengths significantly longer than 850 nm may result in images that are incompatible with expositing iris coding algorithms and databases. Incompatibility with expositing algorithms does not necessarily obviate the usefulness of longer wavelengths, but will nevertheless impact marketability of a longer wavelength solution. The responsivity of inexpensive silicon based detectors also falls very rapidly as wavelengths increase past 850 nm, making detection of long wavelength light increasingly expensive.
With respect to increasing exposure time, the fact that the subject may be moving constrains the illumination time in conventional non-tracking systems. Increasing the illumination time increases the possibility of motion blur.
With respect to increasing the numerical aperture, it is well known that for a given image scale, larger lenses capture more light. There are cost constraints to building very fast lenses, but more significant is the effect on depth of field. Because the subject may be moving, the depth of field of the imaging device has to match the maximum expected depth movement of the subject over the exposure time. If the camera is statically focused, a smaller depth of field also limits the probability that the subject will be within the focus range. Increasing the numerical aperture without increasing the detector pixel pitch, can lead to aliasing errors, where high frequency image structure appears incorrectly as low frequency features. Both effects can be somewhat ameliorated by deliberately reducing the fidelity of the lens.